Planar illumination device

ABSTRACT

A planar illumination device comprises a light guide plate, a point-like light source arranged on an incoming-light face of the light guide plate, an inner frame formed substantially in a U-shape on a top view, and an outer frame having a seat portion and side walls wherein the light guide plate is accommodated in the inner frame and mounted on the seat portion of the outer frame, while the point-like light source is arranged along the incoming-light face of the light guide plate and held between the incoming-light face and the side wall of the outer frame. At the inner frame, an elastic action portion which is elastically deformed so as to make a partial contact with the outer frame and energizes the light guide plate toward the point-like light source is provided.

This is a Continuation of application Ser. No. 11/919,138 filed Nov. 13,2007, which in turn is a National Stage of PCT/JP2006/302846 filed Feb.17, 2006, and which claims priority to Japanese Application No.2005-128488 filed on Apr. 26, 2005. The disclosure of the priorapplications is hereby incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present invention relates to a side-light-type planar illuminationdevice and particularly to a planar illumination device used asilluminating means of a liquid crystal display device.

BACKGROUND ART

As an auxiliary light source for a liquid crystal display device usedfor a cellular phone or the like, a side-light type planar illuminationdevice in which a primary light source is arranged on a side end face ofa light guide plate is mainly used (hereinafter the side end face onwhich the primary light source is arranged is also called as aincoming-light face). As the primary light source of a side-light-typeplanar illumination device, a cold-cathode tube lamp has been used, butcurrently, a point-like light source such as a white LED which is morein impact resistance than the cold-cathode tube lamp is used in manycases. Application fields of the planar illumination device using such apoint-like light source tend to expand, and application not only asmall-sized liquid crystal display device used in a cellular phone orthe like but also as an auxiliary light source of a liquid crystaldisplay device used in an in-vehicle navigator with a relatively largedisplay size, for example, is being discussed.

In order to respond to the expansion of an illumination area, varioustrials are made for increase of an outgoing light amount from theprimary light source and efficient use of the outgoing light. Forexample, the increase of the outgoing light amount is promoted throughincrease of the number of point-like light sources arranged on one sideend face of a light guide plate, arrangement of the point-like lightsource on a plurality of side end faces of the light guide plate orincrease of the outgoing light amount per point-like light source byincreasing an electric current to be supplied to the point-like lightsource.

On the other hand, increase of the point-like light sources constitutingthe primary light source or increase of the electric current to besupplied to each of the point-like light sources accompanies a problemof increase in a heat amount generated from the primary light source,leading to rise of an ambient temperature and lowered light emittingefficiency of the point-like light source. Also, when heat from theprimary light source is transmitted to the light guide plate, positionaldisplacement between the light guide plate and the point-like lightsource is induced by expansion of the light guide plate, which resultsin a problem that efficient use of the outgoing light is prevented andalso causes fluctuation in illumination brightness of the planarillumination device. Moreover, if the light guide plate is accommodatedin a frame, such nonconformity might occur that the light guide plate isdeformed due to a difference in heat expansion coefficient between thelight guide plate and the frame.

In order to efficiently radiate heat generated from the point-like lightsource, such a planar illumination device is proposed in which the lightguide plate and the point-like light source are accommodated in a metalchassis and radiation performance of the point-like light source isimproved by bringing the point-like light source into direct contactwith the metal chassis (See Patent Document 1, for example). Also, aplanar illumination device 100 in which an elastic action portion 120 isprovided at a frame 102 as a method for restraining nonconformity causedby the heat expansion of the light guide plate shown in FIG. 3 isproposed (See Patent Document 2, for example).

In the planar illumination device 100, the elastic action portion 120comprises a thin beam portion 122 capable of elastic deformation formedby providing a thickness reducing hole 123 on a side wall 102 b of theframe 102 and a projection portion 121 projecting from the center of thebeam portion 122. The frame 102 accommodates a light guide plate 103 anda fluorescent lamp 105 held by a lamp holder 104, and the light guideplate 103 is arranged so that the projection portion 121 of the elasticaction portion 120 is brought into contact with a side end face(terminal face) 103 b on the side opposite a side end face(incoming-light face) 103 a on which the fluorescent lamp 105 isarranged. By holding the lamp holder 104 between the incoming-light face103 a and the side wall 102 a of the frame 102 opposite theincoming-light face 103 a, the fluorescent lamp 105 is arranged alongthe incoming-light face 103 a of the light guide plate 103. In theplanar illumination device 100, with the above configuration, the lightguide plate 103 is energized to the fluorescent lamp 105 side with anelastic force F from the elastic action portion 120 and the deformationof the light guide plate 103 is absorbed.

Patent Document 1: Japanese Unexamined Patent Application PublicationNo. 2004-186004 (paragraphs [0035] to [0037], FIG. 3)

Patent Document 2: Japanese Unexamined Patent Application PublicationNo. 2003-338214 (paragraphs [0016] to [0017], FIG. 1)

DISCLOSURE OF INVENTION

However, since the planar illumination device 100 shown in FIG. 3 hasthe projection portion 121 of the elastic action portion 120 broughtinto contact with the terminal face 103 b of the light guide plate 103,a gap g exists between the terminal face 103 b of the light guide plate103 and the side wall 102 b of the frame 102 opposite the terminal face103 b. Therefore, light leaking from the terminal end face 103 b cannotbe efficiently reflected to the light guide plate 103, which causes aproblem of lowered use efficiency of the light. Also, in the planarillumination device 100, the face of the side wall 102 b of the frame102 opposite the terminal face 103 b of the light guide plate 103 hasconfigurations different between the region in which the elastic actionportion 120 is formed and the other regions. That is, the region wherethe elastic action portion 120 of the side wall 102 b is formedgenerally comprises a recess-state curved face formed by flexuraldeformation of the beam portion 122 and the surface of the projectionportion 121 present at its center, which is different from a flat planeconstituting the other regions in the side wall 102 b, and in addition,even if the side wall 102 b and the terminal face 103 b of the lightguide plate 103 are brought into substantially close contact with eachother according to expansion or the like of the light guide plate 103,for example, on both sides of the projection portion 120, there is nodirect contact with the terminal face 103 b but a portion with an airlayer remains therebetween. Such a difference in plane configurationmight cause a difference in light reflectance between both andunevenness in brightness of the planar illumination device.

Moreover, the configuration described in Patent Document 1 presumes useof the fluorescent lamp 105 as the primary light source, and noparticular consideration is given to its radiation performance. On theother hand, in order to use the point-like light source such as thewhite LED as the primary light source and to achieve its higherbrightness, it is preferable to efficiently radiate the heat generatedfrom the point-like light source by bringing the point-like light sourceinto direct contact with the metal chassis as described in PatentDocument 1, for example, but the planar illumination device described inPatent Document 1 does not have an action to stably maintain bondingwith the light guide plate against expansion and contraction due to heatexpansion or the like, and there is room for improvement in that regard.

The present invention was made in view of the above problems and has anobject to provide a planar illumination device capable of efficientlyradiating heat from the point-like light source and stably maintainingbonding between the light guide plate and the point-like light source byabsorbing expansion and contraction of the light guide plate.

In order to achieve the above object, the planar illumination deviceaccording to the present invention comprising a light guide plate, apoint-like light source arranged on a side end face of the light guideplate, and a frame holding them, the frame comprising an inner frameformed substantially in the U-shape on a top view and an outer framehaving a plane-state seat portion and a side wall installed upright onan outer edge portion of the seat portion and accommodating the innerframe, the light guide plate being accommodated in the inner frame andmounted on the seat portion together with the inner frame, thepoint-like light source being held between a side end face on an openside of the inner frame in the light guide plate and the side wall ofthe outer frame opposite the side end face, and the inner frame beingprovided with an elastic action portion which is brought into contactwith the side wall of the outer frame and elastically deformed so as toenergize the light guide plate toward the point-like light source side.

According to the present invention, the frame holding the light guideplate and the point-like light source is in the double structure of theU-shaped inner frame accommodating the light guide plate and the outerframe accommodating the inner frame, while the point-like light sourceis held between the side end face on the open side of the inner frame inthe light guide plate and the side wall of the outer frame opposite theside end face so as to be arranged in direct contact with the outerframe. Thus, by forming the outer frame from a material having heatconductivity higher than that of the inner frame, the heat generatedfrom the point-like light source can be efficiently radiated, whichcontributes to higher brightness of the planar illumination device.

Moreover, since the elastic action portion energizing the inner frametoward the point-like light source is provided at the inner frame in thepresent invention, bonding efficiency between the light guide plateaccommodated in the inner frame and the point-like light source can beimproved, and even if the light guide plate is expanded/contracted in adirection crossing the incoming-light face of the light guide plate(vertical direction), the expansion/contraction is absorbed by elasticdeformation of the elastic action portion and the bonding between thelight guide plate and the point-like light source can be stablymaintained. Also, since the elastic action portion according to thepresent invention is elastically deformed upon contact with the outerframe, the face opposite the side end face of the light guide plate inthree sides constituting the inner frame can be configured as an evenflat face and each of the side end faces of the light guide plateexcluding the incoming-light face can be accommodated in close contactwith those three sides. Therefore, light leaking from the side end faceof the light guide plate can be efficiently returned to the light guideplate without unevenness in the brightness.

Also, since the inner frame according to the present invention is formedin the U-shape and the point-like light source is arranged with the sideend face on the open side of the inner frame in the light guide plate asthe incoming-light face, even if the light guide plate isexpanded/contracted in a direction parallel to the incoming-light face(horizontal direction), an interval between opposing two sides in thethree sides forming the U-shape can be deformed flexibly following thedeformation of the light guide plate by the elasticity of the innerframe itself.

As a mode of the present invention, the elastic action portion accordingto the present invention comprises a thin beam portion provided at aside opposite the open side of the inner frame and a projection portionprojecting outward of the inner frame from substantially the center ofthe beam portion, and the inner frame is accommodated in the outer frameby bringing the projection portion into contact with the side wall ofthe outer frame.

By this configuration, the elastic action portion which is suitable forelastic deformation upon contact with the side wall of the outer frameand energizing the light guide plate toward the point-like light sourcecan be easily formed integrally with the inner frame.

Preferably, the elastic action portion is provided at the opposing twosides of the inner frame, and by providing the elastic action portion atthree sides constituting the inner frame, the inner frame can be stablyheld at the outer frame and the deformation in the 2 axial directions(the above vertical direction and horizontal direction) of the lightguide plate can be both absorbed.

Also, as a mode of the present invention, the elastic action portionaccording to the present invention comprises a cantilever beam providedat opposing sides of the inner frame, respectively, and extending withinclination from one side of the inner frame toward a free end of theopen side of the inner frame so as to expand outward of the inner frame,and the inner frame may be accommodated in the outer frame by bringingthe respective cantilever beams into contact with an upper end of eachside wall opposite the opposing two sides of the inner frame in theouter frame. By constituting the elastic action portion according to thepresent invention as above, the elastic action in the above 2 axialdirections with respect to the light guide plate can be realized by thepair of elastic action portions provided at two opposing sides,respectively, of the inner frame.

Also, the inner frame is formed from a white resin material, while theouter frame is preferably formed from a heat conductive metal material,by which the elastic action portion according to the present inventioncan be easily formed integrally, and the planar illumination device canbe realized, provided with the inner frame capable of efficientlyreflecting the leakage light from the side end face of the light guideplate and returning it to the light guide plate and the outer frameexcellent in radiation performance of the heat generated from thepoint-like light source.

Since the present invention is configured as above, the heat from thepoint-like light source can be efficiently radiated and the bondingbetween the light guide plate and the point-like light source can bestably maintained by absorbing the expansion/contraction of the lightguide plate. Thus, higher and stable brightness of the planarillumination device can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating an essential part of a planar illuminationdevice in a first embodiment of the present invention, in which FIG. 1Ais a top view and FIG. 1B is a side view illustrating a part as an A-Asection;

FIG. 2 is a top view illustrating an essential part of the planarillumination device in a second embodiment of the present invention; and

FIG. 3 is a top view illustrating a configuration example of aconventional planar illumination device.

REFERENCE NUMERALS

10, 30 planar illumination device

11 outer frame

12, 32 inner frame

13 light guide plate

15 point-like light source

20 elastic action portion

21 projection portion

22 beam portion

33 elastic action portion (cantilever beam)

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described below referringto the attached drawings, but each drawing is for explanation and doesnot necessarily reflect actual shapes or dimensions accurately.

FIG. 1 is a view illustrating an essential part of a planar illuminationdevice 10 in a first embodiment of the present invention, in which FIG.1A is a top view and FIG. 1B is a side view illustrating a part as anA-A section. The planar illumination device 10 in this embodimentcomprises a light guide plate 13, a point-like light source 15 arrangedon a side end face 13 a of the light guide plate 13, and frames 11, 12holding them, and the frame comprises an inner frame 12 formed in theU-shape on a top view and an outer frame 11 having a plane-state seatportion 11 e and side walls 11 a, 11 b, 11 c, 11 d installed upright onan outer edge portion of the seat portion 11 e. In the planarillumination device 10, the light guide plate 13 is accommodated in theinner frame 12 and mounted on the seat portion 11 e of the outer frame11 together with the inner frame 12, and the point-like light sources 15are arranged along the side end face (incoming-light face) 13 a on theopen side of the inner frame 12 in the light guide plate 13 and are heldbetween the incoming-light face 13 a and the side wall 11 a of the outerframe 11 opposite the incoming-light face 13 a. Also, in thisembodiment, elastic action portions 20,20′ are provided at three sides12 b, 12 c, 12 d constituting the inner frame 12 substantially in theU-shape, and detailed configuration and actions of the elastic actionportions 20, 20′ will be described later.

Here, the light guide plate 13 is a plate-state light guide body formedby molding a transparent resin such as an acrylic resin or apolycarbonate resin into a substantially rectangular shape, and thelight incident from the incoming-light face 13 a is evenly emitted fromthe surface 13 a while it is propagated into the light guide plate 13with the upper face 13 e as the light emitting face and the lower face13 f as the reflecting face.

Also, the point-like light source 15 is made from a white LED, forexample, and is arranged with the light emitting face toward theincoming-light face 13 a of the light guide plate 13. Also, though notshown, the point-like light sources 15 are mounted on a circuit boardsuch as a flexible print circuit board and the circuit board may be, forexample, fastened to the upper face 13 e or the lower face 13 f of thelight guide plate 13 or the side wall 11 a of the outer frame 11.

In this embodiment, the inner frame 12 has two opposing sides 12 c, 12 dand the bottom side 12 b connecting one ends of the two sides and isformed substantially in the U-shape with the side opposite the bottomside 12 b as the open side and molded from a white resin such as apolycarbonate resin in which titanium oxide as a white pigment, forexample, is mixed. The outer frame 11 is formed from a metal materialwith high heat conductivity such as aluminum and has the plate-stateseat portion 11 e and side walls 11 a, 11 b, 11 c, 11 d installedupright on the outer edge portion of the seat portion 11 e by usualpress machining or the like.

The planar illumination device 10 has the double-structured frameconstituted by the inner frame 12 and the outer frame 11 as above, andthe light guide plate 13 is accommodated in the inner frame 12 with thethree side end faces 13 b, 13 c, 13 d, except the incoming-light face 13a, in close contact with the corresponding three sides 12 b, 12 c, 12 dof the inner frame 12, and the integrated light guide plate 13 and theinner frame 12 are accommodated in the outer frame 11 by mounting themonto the seat portion 11 e of the outer frame 11. At this time, the sideend faces 13 b to 13 d of the light guide plate 13 and the correspondinginner faces of the sides 12 b to 12 d of the inner frame 12 are notfastened, and the light guide plate 13 and the inner frame 12 areintegrated so that relative displacement between the side end faces 13c, 13 d of the light guide plate 13 and the corresponding sides 12 c, 12d of the inner frame is possible with expansion/contraction of the lightguide plate 13.

In this embodiment, at the bottom side 12 b of the inner frame 12, thepair of elastic action portions 20 which are elastically deformed uponcontact with the side wall 11 b of the outer frame 11 and energizes theinner frame 12 (thus, the light guide plate 13 accommodated integrallyin the inner frame 12) toward the point-like light source 15 side areformed, and elastic action portions 20′ having the same configuration asthe elastic action portions 20 and elastically deformed upon contactwith the side walls 11 c, 11 d of the outer frame 11, respectively, areformed at the opposing two sides 12 c, 12 d opposite the inner frame 12.

The configuration and actions of the elastic action portions 20, 20′will be described below, in which the direction parallel to theincoming-light face 13 a of the light guide plate 13 is referred to as ahorizontal direction, the direction crossing the incoming-light face 13a as a vertical direction and the directions in vertical and horizontaldirections of the planar illumination device 10 are definedcorresponding to the vertical and horizontal directions of the figure onpaper as necessary.

First, the elastic action portion 20 comprises a thin beam portion 22capable of elastic deformation and formed by providing a through hole 23substantially parallel to the extending direction on the bottom side 12b of the inner frame 12 and a projection portion 21 projecting outwardof the inner frame 12 from substantially the center of the beam portion22. In this embodiment, the inner frame 12 is arranged so that theprojection portion 21 of the elastic action portion 20 is brought intocontact with the side wall 11 b of the outer frame 11 and the beamportion 22 is slightly bent inward of the inner frame 12, by which adrag F_(U) upward in the vertical direction acts on the bottom side 12 bof the inner frame 12 from the side wall 11 b as a reaction to theelastic force exerted by the elastic action portion 20 on the side wall11 b. By this drag F_(U), the inner frame 12 and the light guide plate13 are energized toward the point-like light source 15 side, and thepoint-like light source 15 held between the incoming-light face 12 a ofthe light guide plate 13 and the side wall 11 a of the outer frame 11opposite the incoming-light face 12 a is favorably and stably bonded tothe light guide plate 13, and efficient heat radiation is promoted bycontact with the side wall 11 a from the heat conductive metal material.Also, even if the light guide plate 13 is expanded/contracted byfluctuation of the ambient temperature or the like, the verticalexpansion/contraction is absorbed by the elastic deformation of the beamportion 22 of the elastic action portion 22.

On the other hand, the elastic action portion 20′ is formed by providinga structural body similar to the elastic action portion 20 at theopposing two sides 12 c, 12 d of the inner frame 12 substantially inparallel with their extending direction. In this embodiment, the innerframe 12 is arranged so that the projection portion 21 of the elasticaction portion 20′ is brought into contact with the side walls 11 c, 11d of the outer frame 11 and the beam portion 22 is slightly bent inwardof the inner frame 12, by which the drag F_(R) to the right in thehorizontal direction from the side wall 11 c acts on the side 12 c ofthe inner frame 12 as a reaction to the elastic force exerted by theelastic action portion 20′ on the side wall 11 c and the drag F_(L) tothe left in the horizontal direction from the side wall 11 d on the side12 d of the inner frame 12 as a reaction to the elastic force exerted bythe elastic action portion 20′ on the side wall 11 d, respectively. Theinner frame 12 is stably held by the outer frame by the drag F_(R) andthe drag F_(L). Also, since the inner frame 12 is formed in the U-shape,the interval between opposing two sides 12 c, 12 d is varied flexiblyfollowing the deformation of the light guide plate 13 by the elasticityof the inner frame 12 itself, and an expansion/contraction component inthe horizontal direction in the expansion/contraction of the light guideplate 13 can be absorbed by the elastic deformation of the beam portion22 of the elastic action portion 20′.

Since the elastic action portions 20, 20′ are formed on the outer faceside of each of the sides 11 b, 11 c, 11 d constituting the inner frame12 in the planar illumination device 10 as above, the inner face side ofeach of the sides 11 b, 11 c, 11 d of the inner frame 12 can be formedin the shape according to each of the side end faces 13 b, 13 c, 13 d ofthe light guide plate 13 (evenly flat plane, for example), and asmentioned above, each of the side end faces 13 b, 13 c, 13 d can beaccommodated in close contact with each of the sides 11 b, 11 c, 11 d ofthe inner frame 12 so that the leakage light from each of the side endfaces 13 b, 13 c, 13 d can be efficiently reflected and returned to thelight guide plate 13 without causing unevenness in the brightness.

The elastic action portions 20, 20′ are preferably molded integrallywith the inner frame 12 made from a white resin and at that time, thethickness and length of the beam portion 22 and the shape/dimension ofthe through hole 23 or the number, positions or the like of the elasticaction portions 20, 20′ are appropriately set so that theexpansion/contraction of the light guide plate 13 is absorbed and thedrags F_(U), F_(R), F_(L) appropriate for the inner frame 12 acts.

Next, the planar illumination device in a second embodiment of thepresent invention will be described. FIG. 2 is a top view illustratingan essential part of the planar illumination device 30. The planarillumination device 30 in this embodiment has configuration basicallysimilar to that of the planar illumination device 10 shown in FIG. 1,and only the configuration of the elastic action portion 33 provided atthe inner frame 32 is different. Thus, description on the duplicatedportions will be omitted and the configuration and actions of theelastic action portion 33 will be described in detail.

In the planar illumination device 30, the elastic action portion 33comprises a pair of cantilever beams 33 provided at opposing two sides32 c, 32 d of the inner frame 32, the cantilever beam 33 extending withinclination from each of the sides 32 c, 32 d of the inner frame towarda free end 33 a on the open side of the inner frame 32 so as to expandoutward of the inner frame 32, and at the free end (distal end), anengagement portion 33 a to be engaged with upper end portions 14 c, 14 dof the side walls 11 c, 11 d of the outer frame 11 is provided.

In this embodiment, the inner frame 32 is arranged so that the elasticaction portion 33 is brought into contact with the side walls 11 c, 11 dof the outer frame 11, and the inclination is slightly bent inward ofthe inner frame 32, by which a drag F directed diagonally upward in theright from the side wall 11 c acts on the side 32 c of the inner frame32 as a reaction to the elastic force exerted by the elastic actionportion 33 on the side wall 11 c, and a drag F′ directed diagonallyupward in the left from the side wall 11 d acts on the side 32 d of theinner frame 32 as a reaction to the elastic force exerted by the elasticaction portion 33 on the side wall 11 d. In this embodiment, the innerframe 32 and the light guide plate 13 are energized by an upwardcomponent F_(U) of the drags F, F′ toward the point-like light source 15with the respective horizontal components F_(R), F_(L) acting at thesame time, which stably hold the inner frame 32 at the outer frame 11.Similarly, the expansion/contraction in the vertical direction and thehorizontal direction of the light guide plate 13 caused by fluctuationin the ambient temperature or the like is absorbed by fluctuation in theinclination of the elastic action portion 33 outward or inward of theinner frame 32. The planar illumination device 30 obtains the sameactions/effects as those of the planar illumination device 10 in theabove-mentioned first embodiment by the pair of elastic action portions33 provided at the opposing two sides 32 c, 32 d of the inner frame 32.

1. A planar illumination device provided with a light guide plate;point-like light sources arranged on a side end face of the light guideplate; and a frame holding the light guide plate and the point-likelight sources, wherein the frame comprises an inner frame formedsubstantially in a U-shape in the plane of the light guide plate and anouter frame having a plate-state seat portion and a plurality of sidewalls is installed upright on an outer edge portion of the seat portionso as to accommodate the inner frame; the light guide plate isaccommodated in the inner frame and mounted on the seat portion togetherwith the inner frame; the point-like light source is held between theside end face on an open side of the inner frame in the light guideplate and at least one of the plurality of side walls of the outer framethat is opposite the side end face; and an inner face side of each sideconstituting the inner frame is formed in a shape according to each sideend face of the light guide plate, and on an outer face side of at leastone of the sides constituting the inner frame, an elastic action portionis provided that is elastically deformed by making partial contact withat least one of the plurality of side walls of the outer frame andenergizing the light guide plate toward the point-like light source. 2.The planar illumination device according to claim 1, wherein the elasticaction portion is provided on one side of the inner frame opposite theopen side of the inner frame and comprises a thin beam portion and aprojection portion projecting outward of the inner frame substantiallyfrom the center of the beam portion, and the inner frame is accommodatedin the outer frame with the projection portion in contact with at leastone of the plurality of side walls of the outer frame.
 3. The planarillumination device according to claim 2, wherein the elastic actionportion is further provided at opposing two sides of the inner frame. 4.The planar illumination device according to claim 1, wherein the innerframe is formed from a white resin material and the outer frame isformed from a heat conductive metal material.
 5. The planar illuminationdevice according to claim 2, wherein the inner frame is formed from awhite resin material and the outer frame is formed from a heatconductive metal material.
 6. The planar illumination device accordingto claim 3, wherein the inner frame is formed from a white resinmaterial and the outer frame is formed from a heat conductive metalmaterial.